As an electrolyte solution for electrochemical device, a nonaqueous electrolyte solution for electrochemical capacitor comprising a solution of a cyclic amidinium salt in propylene carbonate as a solvent has been known (Patent Document 1 and Patent Document 2).
Conventional nonaqueous electrolyte solutions do not have sufficient electric strength in some cases, so that an electrochemical device, especially an electrochemical capacitor, which contains such electrolyte solution sometimes causes marked time degradation in its performance.    Patent Document 1: International Publication WO95/15572    Patent Document 2: Japanese Kokai Publication 2005-197666